Fissionable fuel materials such as oxides of uranium, plutonium or thorium, and combinations thereof, are typically formed into small cylindrical slugs or pellets and housed within sealed tubes or elongated containers sometimes referred to in the art as "cladding". The cladding protects the fuel from reacting with the coolant, or any foreign matter entrained therein, and prevents escape of any fission products, which are normally highly radioactive and corrosive, from the fuel into the coolant, thereby preventing contamination of the overall system. Thus the enduring integrity of the sealed cladding housing the fissionable fuel is crucial.
Large-capacity power-generating nuclear fission reactor plants normally have several hundred sealed cladding tubes for housing fissionable fuel. To facilitate periodic refueling, which commonly is performed by replacing fractional portions of the total fuel at intervals and rearranging other fractional portions, these fuel rods or pins are conventionally assembled into bundles or groups of elements which can be handled and manipulated as a single composite unit.
The fuel rods of each bundle are held mutually parallel and spaced apart by mechanical means. A typical fuel bundle comprises, for example, an 8.times.8 or 9.times.9 array of spaced fuel rods. The cladding is usually more than 10 ft long, e.g., 14 ft, and approximately 1/2 inch in diameter the cladding tubes being spaced from each other by a fraction of an inch. The spacing is required to permit an ample flow of heat-removing coolant, such as water, over the full exterior surface of the cladding for effective heat transfer and thus effective reactor operation.
To inhibit the fuel rods from bowing and vibrating due to high heat and high velocities of coolant flowing thereabout, which could cause adjacent fuel rods to contact and in any case could impede or unbalance coolant flow, the fuel rods are retained in a spaced-apart array or relation by means of a plurality of spacing grids (hereinafter "spacers") positioned at intervals along the fuel rod length.
A typical spacer comprises a plurality of parallel cells which are welded to each other and to a surrounding spacer band to form a lattice of cells. Each cell receives one fuel rod. Each fuel rod passes through a plurality, e.g., seven, of spacers. These spacers are mutually aligned and spaced along the length of the fuel rods. Each spacer receives a different axial portion of the plurality of fuel rods making up the fuel bundle. The spacers provide intermediate restraint and support transverse of the fuel rods, thereby preventing lateral bowing and vibration which could damage the fuel rods or impede effective coolant flow intermediate and around each fuel rod. Spacers for securing bundles of fuel rods often incorporate spring and stop members which press against the fuel rods in metal-to-metal contact as a means of securely gripping and holding the fuel rods in position. The fuel rods additionally have their ends supported in respective sockets of upper and lower tie plates.
The fuel rod bundle assembly is also typically surrounded by an open-ended tubular fuel channel of suitable cross section, e.g., square. The fuel channel directs the flow of coolant longitudinally along the surface of the fuel rods and guides the neutron-absorbing fission control rods which reciprocate longitudinally intermediate adjacent bundles.
Structural components used within the reactor core of fissionable fuel must be fabricated from a durable metal which has a low neutron-absorbing capacity or cross section, so as not to impede the neutron-incited fission chain reaction. The preferred materials most commonly used comprise alloys of zirconium which have a neutron absorption capacity on the order of about one-fifteenth that of stainless steel. However, under certain circumstances zirconium alloys are susceptible to corrosion which can result in structural failure. To impede a destructive form of self-perpetuating corrosion peculiar to zirconium and its alloys referred to in the art as nodular corrosion, components produced from zirconium alloys, such as cladding, are commonly treated to form a specific oxide surface layer which resists nodular corrosion and surface attack under reactor conditions.
Referring to FIG. 1, a typical nuclear fuel bundle 10 comprises a group of spaced-apart, mutually parallel fuel rods 12. Each fuel rod comprises a cylindrical container 14 (i.e., cladding) which houses a vertical stack of pins or slugs of fissionable fuel (not shown) sealed therein. Each fuel rod 12 is transversely secured in the parallel array by a series of spacers 16 positioned at intervals along the length of the fuel rods. The ends of the fuel rods of each bundle are fixed within respective sockets of upper and lower tie plates 18 and 20. The bundle assembly of grouped fuel rods 12 and spacers 16 is surrounded by an open-ended fuel channel 22.
It has been determined that abrasions or damage such as scratches in the surface of metal fuel rods present potential sites for the subsequent occurrence of destructive forms of corrosion. For example, surface abrasions or scratches in zirconium alloys render the site susceptible to a form of corrosion which can result in progressive erosion when exposed to the aggressive thermal and chemical environment of a nuclear reactor fuel core. This progressive form of corrosion is called "nodular corrosion" because it occurs as a deeply penetrating area of erosion producing a white oxide surface nodule. Nodular corrosion can significantly impair the structural integrity of the zirconium alloy cladding to the degree of rendering the cladding vulnerable to rupture. Rupture of the cladding results in leakage of radioactive fission products from the fuel rod into the recirculating coolant, which carries the contaminants throughout the system, and the entry of coolant and any entrained impurities into the fuel rod and into contact with the fissionable fuel.
Evaluations have shown that surface abrasions and damage such as scratches are primarily inflicted during assembly of the fuel bundles. In particular, abrasion and scratching occurs when the spring and stop members of the spacer cells bear against the moving surface of the cladding as each fuel rod passes through successive spacers. Secondarily, abrasion and scratching can occur during preassembly handling of the cladding, e.g., if one cladding element rubs against another cladding element or contacts storage and production structural members, or during shipping and handling prior to installation in the reactor fuel core.
As each fuel rod is inserted into the series of spacers during fuel bundle assembly, the number of axial scratches increases as the fuel rod progresses through each successive spacer. Each section of fuel rod can receive a number of scratches equal to the product of the number of contact points for each spacer cell and the number of spacers the fuel rod passes through. For example, if each cell has three contact points and if the assembly has seven spacers, then the leading section of the fuel rod potentially could receive 24 scratches depending on rotation of the fuel rod.
Scratches in the surface of the cladding upset the surface topography which has been treated for optimal corrosion resistance prior to fuel bundle assembly. It is highly desirable to minimize damage to the cladding surface. However, conventional devices and methods for preventing scratches during insertion of fuel rods into spacers have various drawbacks.
It is known to coat the cladding surface with lubricants prior to insertion of the fuel rod. However, lubricants have been found to be ineffective. Varnish type coatings have also been used. Although such coatings are effective, the varnish must be removed after assembly by chemical means. Disposal of the varnish and varnish remover is expensive.
In the prior art, plastic sleeves having a longitudinal slit are inserted into the spacer cells to prevent sliding contact between the fuel rods and the spacer cells during fuel rod insertion. Individual sleeves are attached to individual holders. During fuel bundle assembly, insertion and removal of sleeves is done one rod at a time. The first fuel rod is inserted through all spacers and the sleeves are removed. Then the process is repeated for the next fuel rod. In order to remove the sleeves of last few fuel rods, the fuel rods must be inserted in the central region of the bundle first. The sleeve at each spacer is removed by sliding the sleeve longitudinally until the sleeve is clear of the spacer and then opening the sleeve to allow the fuel rod to pass radially through the expanded slit. This process increases the cost of fuel bundle assembly.
U.S. Pat. No. 4,740,351 to Katsumizu et al. discloses an arrangement which utilizes flexible sleeves to separate the fuel rods from abutting parts of the spacer cells during assembly. The protective sleeve is formed by winding flexible sheet material, the opposing edges of the sheet defining a longitudinal slit for removing the fuel rods. The sleeve material is polyester film. The sleeve has a length substantially equal to the overall length of the fuel rod. This patent also states that Japanese Patent Publication No. 11244/1978 discloses a stainless steel protecting tube about 0.1 mm thick fitted around the fuel rod.
U.S. Pat. No. 4,800,061 to Shallenberger et al. discloses a thin-walled metallic sleeve having a longitudinal slit. The sleeve serves as a protective envelope for the fuel rod during insertion of the fuel rod into the spacer cells. The sleeve has a uniform wall thickness of 0.006 to 0.008 inch and is preferably made of stainless steel.